Ultranarrow bandwidth Faraday atomic filter approaching natural linewidth based on cold atoms

An ultranarrow bandwidth Faraday atomic filter is realized based on cold Rb87 atoms. The atomic filter operates at 780 nm on the 52 S 1/2, F=2 to 52 P 3/2, F′=3 transition with a bandwidth of 7.1(8) MHz, which is approaching the natural linewidth of the transition. The peak transmission achieves 2.6(3)% by the multi-pass probe method. This atomic filter based on cold atoms may find potential applications in self-stabilizing lasers in the future.